The present invention generally relates to an automatic focus control system with a backlash correcting arrangement for a photographic camera and more particularly, to an automatic focus control system which is so arranged that, by detecting, through photoelectric measurements, electrical calculating processing, etc., an amount of deviation in the position of an image of a target object formed by an objective lens with respect to a focal plane, i.e, the so-called amount of deviation in the focusing, a driving amount of a motor for driving a focusing optical system (i.e., optical system to be displaced for the focus control) of the objective lens is calculated so as to control the driving motor based on the result of the calculation for the focus control of the objective lens.
Up to the present, there has been proposed by the present inventors, for example, in Japanese Laid-Open Patent Application (Tokkaisho) No. 57-93326, an automatic focus control system in which distribution of light intensity or incident light amount of a target object light transmitted through an objective lens on a predetermined focal plane is photoelectrically detected so as to obtain data of a deviation amount of an image of the target object formed by the objective lens with respect to the predetermined focal plane, by subjecting the result of the detection to a predetermined calculation processing for driving a driving motor based on the above data, whereby a focusing optical system of the objective lens is driven through a driving mechanism coupled to the driving motor for effecting the focus control.
In the known arrangement as described above, within a mechanical transmission system ranging from the driving motor to the focusing optical system, there is a backlash or play between elements such as gears, connecting members, etc. related to the transmission, and it is impossible to completely eliminate such backlash or play in the arrangement. In the case where the focus control is effected through the transmission system as described above, there is no problem if the focusing optical system reaches an in-focus position while the driving motor is rotating in one direction and the driving of the motor is stopped thereat, but when the focusing optical system is subjected to reciprocating movements before arrival at the in-focus position, for example, in the case where the focusing optical system has undesirably passed through the in-focus position by some causes and it is required to return the focusing optical system by reversing the driving motor, an error equivalent to the backlash is produced during the reversing of the driving direction. More specifically, in the above case, with respect to a certain rotational amount of the driving motor, the focusing optical system is to be driven less than a predetermined driving amount by the backlash amount. As a result, in the case where the focusing optical system is caused to arrive at the in-focus position by repeating the focus detection (i.e., detection of an amount of deviation in the position of an image of the target object to be photographed with respect to the predetermined focal plane), while moving the focusing optical system of the objective lens, there may be such inconveniences that the focusing optical system repeats reciprocating movements in the vicinity of the in-focus position or it takes a long time for the optical system to reach the in-focus position. Meanwhile, in such a case where the focus detection is effected, with the focusing optical system remaining stationary, and based on the result of the above focus detection, the focusing optical system is moved up to a position in the vicinity of the in-focus position for a rough adjustment, and subsequently, another focus detection is effected again so as to effect a fine adjustment of the position of the focusing optical system based on said another focus detection, an error is produced in the amount of adjustment during the fine adjustment if the driving direction of the focusing optical system is altered between the rough adjustment and the fine adjustment. Moreover, also in the case where the focusing control is effected based on the focus detection of only one time, when the focusing is effected with respect to another target object after one target object has been brought into focus, an error is produced in a second adjusting amount, if the moving direction of the focusing optical system is changed.